Insulting someone is not necessarily an instance of an ad hominem fallacy. For example, if one supplies sufficient reasons to reject an interlocutor's argument and adds a slight character attack at the end, this character attack is not necessarily fallacious.
Equally an Ad hominem that isn't an insult when it is True
Canadian academic and author Douglas N. Walton has argued that ad hominem reasoning is not always fallacious, and that in some instances, questions of personal conduct, character, motives, etc., are legitimate and relevant to the issue,[26] as when it directly
involves hypocrisy, or actions contradicting the subject's words.
Wiki
Citation please.
So, if someone says, "I think you're a lying dog-faced pony-soldier", that's NOT an ad hominem attack in your opinion because it's "TRUE" (that person is presumably sincere)??
The key problem here is that character is QUALITATIVE (GNOSIS).
The other key problem here is that motives are QUALITATIVE (GNOSIS).
And you seem to be glossing-over the point that even your source specifies "in SOme instances" (probably when the question tautologically involves questions of "character" and or "motive", of course under those conditions "character" and or "motive" would necessarily be relevant). HOWever, "character" and "motive" are both beyond our epistemological limits (un-quantifiable, self-reported, implicit, subconscious).
And your bald assertion that an ad hominem attack is not fallacious if it's "true" is not supported by any of your quoted sources.
The key problem here is that character is QUALITATIVE
Because there is no way to QUANTIFY "character".
And without QUANTIFICATION there is no way to verify a claim's "TRUTH-VALUE".
Your quote "explains" no such thing.
It simply asserts that in SOME cases, presumably if the subject at hand is specifically about a person's history and or personal choices, data relevant to such an inquiry CANNOT be considered off-limits.
HOweVer, I can't imagine a case where a person's history and or personal choices would be a subject of scrutiny wholly divorced from any explicit or implicit attack or endorsement of their abilities and or ideas WHOLESALE.
A "positive" ad hominem is just as fallacious as a "negative" ad hominem.
You shouldn't believe someone just because they're a doctor.
You shouldn't disbelieve someone just because they're a commie.
Well that's encouraging.
The only "problem" being that you can never know "their character" or "motivations" etc, due to your EPISTEMOLOGICAL LIMITS.
And even the most morally repulsive, malevolent person can still make a perfectly valid logically sound statement.
Their moral repulsiveness and malevolence do not magically invalidate their logic.
I admire your faith in sample-bias.
Please teach me to "Judge Correctly" without logic.